Silicone engineered anisotropic lithography for ultrahigh-density OLEDs

Ultrahigh-resolution patterning with high-throughput and high-fidelity is highly in demand for expanding the potential of organic light-emitting diodes (OLEDs) from mobile and TV displays into near-to-eye microdisplays. However, current patterning techniques so far suffer from low resolution, consecutive pattern for RGB pixelation, low pattern fidelity, and throughput issue. Here, we present a silicone engineered anisotropic lithography of the organic light-emitting semiconductor (OLES) that in-situ forms a non-volatile etch-blocking layer during reactive ion etching. This unique feature not only slows the etch rate but also enhances the anisotropy of etch direction, leading to gain delicate control in forming ultrahigh-density multicolor OLES patterns (up to 4500 pixels per inch) through photolithography. This patterning strategy inspired by silicon etching chemistry is expected to provide new insights into ultrahigh-density OLED microdisplays.

1. The authors claimed that their silicone-incorporated organic light-emitting semiconductors (SI-OLES) have strong anisotropic etching profiles like silicon due to EBL on the sidewall. Even though there are many characterization results to prove the mechanism of anisotropic lithography, it is hard to confirm the impact of EBL in micropatterning except SEM images of one rectangle-shaped micropattern in Fig. 1d and Fig. S7.
It is better to show a quantitative comparison of horizontal etch rate and vertical etch rate between SI-OLES and control polymers.
Also, for discussion on the tapered angle of the micropattern, it is better to show more images to prove their high uniformity and yield for micropatterning.
2. The SI-OLES showed even improved EL performance compared to the pristine polymers. Why did they show the improvement? It is better to add the reason and related discussion.
3. When I checked the micropatterns of blue layers which do not have silicone-engineering in Fig.  S17 -19, the patterning results were not so different with red and green SI-OLES layers. It would be interesting to do supplementary experiments or characterization to emphasize the strong anisotropic etching in the manuscript.
Reviewer #3 (Remarks to the Author): The manuscript entitled "Silicone engineered anisotropic lithography for ultrahigh-density OLEDs" submitted by D. H. Kim and coworkers presented intriguing etching characteristics of siliconeincorporated organic light-emitting semiconductors (SI-OLES), enabling to be delicately pixelated by conventional photolithography and reactive ion etching. The approach that unique etching behavior of silicon is implanted on the OLED materials is particularly noteworthy, and this technique seems to significantly contribute improvement of pattern resolution in the field of the OLEDs. Also, the authors systematically described the non-volatile blocking layer that is induced during etching of the SI-OLES, as well as the developed high density RGB OLED pattern arrays and the light-emitting performance were quite impressive. In addition, the manuscript is well-written, and the figures are well-organized. I recommend this high quality manuscript for publication in Nature Communications after minor revision, and several points should be addressed before publication.
1. The authors mentioned that for analysis of non-volatile etch-blocking layer generated on the surface regime of the SI-OLES film, the lowest RF power of RIE was deliberately used. However, as the RF power is too low, the etching performance of RIE can be extremely reduced because ion bombardment effect is not efficiently developed. How can it confirm that the variation of surface modulus of the SI-OLES is the results from the etching reaction of RIE? Also, the authors should indicate the detailed RIE condition.
3. Does the measured contact angle of the SI-OLES film correspond to either R-SI-OLES or G-SI-OLES? This is not clear in the manuscript.
4. The author mentioned that the used Blue-OLES in this work was not incorporated with silicone network because the luminescence performance of the SI-B-OLES was too low, and this problem can be solved if high efficient Blue-OLES is exploited. However, even if high efficient blue materials are developed, it could be meaningless if the chemical robustness of bluish semiconducting materials is not demonstrated when silicone network is adapted. At least, the authors should provide additional data that even blue materials can possess high tolerance when silicone network is integrated. 5. Several typos should be corrected. Response: We thank the reviewer for the invaluable comments. We revised the manuscript according to the reviewer's comments.

Q1)
The pixelated OLEDs for all the colors perform quite poorly and authors have not represented the efficiency metrics of the devices.

Response:
We appreciate the reviewer's comment. We understand the reviewer's concern related to the device performance. As the reviewer commented, light-emitting characteristics of pristine OLEDs, SI-OLEDs, and micro-patterned SI-OLEDs are summarized in Supplementary Table R1. Our SI-OLEDs might be difficult to utilize as well-defined high efficiency displays. The luminance efficiency is, however, comparable or even better to recently presented, non-patterned light-emitting polymer-based OLEDs (Adv. . 2022, 34, 2201844;Sci. Adv. 2021, 7, eabd9715;Angew. Chem. Int. Ed. 2021, 60, 7220-7226), even when our SI-OLEDs underwent micro-patterning fabrication. In this regard, we note that although the SI-OLEDs are micro-pixelated, the light-emitting performance is not inferior to that of the non-patterned OLEDs.

Mater
Furthermore, currently commercialized OLED displays exhibit maximum luminance values of 500 to 1000 cd/m 2 (Specifically, high dynamic range (HDR) of OLED TVs and iPhone 14 OLED displays are 1,000 and 800 cd/m 2 , respectively) (Adv. Funct. Mater. 2021, 31, 2009336;Opt. Express 2017, 25, 33643-33656), which is the level that the demonstrated SI-OLEDs can sufficiently satisfy in terms of maximum luminance. Therefore, we think that the light-emitting capability of the SI-OLEDs can be reasonably acceptable in the field of OLEDs, and the SI-OLEDs possess considerable potential for practical usage.
To facilitate the readers to figure out the OLED characteristics clearly, we added this OLEDs via photolithography. In fact, as an obvious next step, we are currently investigating the silicone engineered anisotropic lithography on phosphorescent small-molecular host-dopant system (see below for details in the reviewer's following comment). We hope this following work can be finalized in the near future.
The suggested multilayered architecture would be beneficial for the enhancement of luminance efficiency of the SI-OLEDs (Nature 2022, 603, 624-630; Nat. Commun. 2015, 5, 5756). As the reviewer pointed out, we emphasize that multilayered SI-OLEDs can be effortlessly achieved, because the chemical 3 robustness of the SI-OLES is directly exploited through successive film deposition processes to form multilayers. Absolutely, the multilayered structure of the SI-OLEDs should be implemented to optimize their light-emitting performance, and this will be carried out near future study.
Despite all the promises, we believe that the scientific novelty and main point of this manuscript are to present a new concept of silicone engineered anisotropic lithography of OLEDs and systemically investigate the non-volatile EBL-induced etching mechanism. can be directly linked to silicone networks (Please note that detailed molecular structures cannot be described due to confidential issues). Based on the small materials, we confirmed that not only physicochemical robustness can be obtained even in the small molecule system, but also fine micropatterns can be achieved by reactive ion etching (RIE)-coupled photolithography ( Figure R1). Therefore, we carefully appeal that the silicone engineered anisotropic lithography can be effectively adaptable to phosphorescent small molecules considered as standard OLED materials, which will be addressed in our further work.

Reply to Reviewer #2
The authors introduced a high-resolution patterning of light-emitting polymer using silicon-engineered lithography for the fabrication of organic light-emitting diodes (OLEDs Response: We appreciate the positive and constructive comments from the reviewer. We revised the manuscript according to the reviewer's comments.

Q1) The authors claimed that their silicone-incorporated organic light-emitting semiconductors (SI-OLES)
have strong anisotropic etching profiles like silicon due to EBL on the sidewall. Even though there are many characterization results to prove the mechanism of anisotropic lithography, it is hard to confirm the impact of EBL in micropatterning except SEM images of one rectangle-shaped micropattern in Fig. 1d and Fig. S7. It is better to show a quantitative comparison of horizontal etch rate and vertical etch rate between SI-OLES and control polymers. Also, for discussion on the tapered angle of the micropattern, it is better to show more images to prove their high uniformity and yield for micropatterning.

Response:
We thank the reviewer's comments. We totally agree that to prove the concept of siliconeinduced anisotropic etching behavior of the SI-OLES more clearly, additional analysis and quantitative assessment of the resulted anisotropic patterns should be discussed.

Development of anisotropic etching profiles indicates that the horizontal etching behavior is
significantly suppressed compared to the vertical one, resulting in considerable reduction of inevitable side-etching properties underneath defined photoresist-patterns ( Figure R2a). By utilizing this phenomenon, horizontal etch rates can be approximately calculated by evaluating the difference of width between the desired and the real patterns. As shown in Figure R2b, the estimated horizontal etch rates of the R-and G-SI-OLES decreased by 61.6 and 61.87 %, respectively, compared to that of R-and G-OLES.
We note that the reduction of horizontal etching behavior is the key evidence of the augmented anisotropism of the SI-OLES (Figure 1a,b). Moreover, this enhanced anisotropic etching behavior can greatly increase a pattern fidelity due to effective alleviation of the side-etching profiles which induce undesired variation of pattern width and severe distortion of pattern edge roughness ( Figure R3a). As shown in Figure R3b and R3c, the pattern width variation and edge roughness of R-and G-SI-OLES were significantly improved. This indicates that 6 the SI-OLES can achieve more reliable fine patterns by exploiting its anisotropic etching behavior than the pristine OLES. In conjunction with the evaluation of the pattern fidelity, second-harmonic generation (SHG) spectroscopy is a powerful tool to confirm the degree of physical defection on the side-wall regime of the patterns. The non-linear SHG signal intensities of the R-and G-OLES patterns were higher than that of the R-and G-SI-OLES (Supplementary Fig. 11), implying that high uniformity in the side-wall regime of the SI-OLES patterns was obtained. Consequently, we believe that the systemically conducted investigations and in-depth discussions are sufficient for the confirmation of the anisotropic etching behavior of the SI-OLES.
To more clearly reveal and elucidate the anisotropic etching behavior of the SI-OLES, we added the Figure R2 and Figure R3 as Supplementary Figure 12 and Supplementary Figure 10, respectively, and the following sentence on the page 6 in the revised manuscript.

"To investigate the precision of the SI-OLES-based micropatterns in details, we quantitatively evaluated
the key assessment factors of pattern fidelity, such as variation of pattern width and degree of pattern edge roughness ( Supplementary Fig. 10).

The width variation and edge roughness of the SI-OLES-based micropatterns exhibited much lower values compared to those of the pristine OLES, indicating that inevitable side-etching phenomenon was effectively suppressed in the SI-OLES."
Q2) The SI-OLES showed even improved EL performance compared to the pristine polymers. Why did they show the improvement? It is better to add the reason and related discussion.

Response:
We appreciate the reviewer's comment. An initial mixing solution of the polymers and silicone network precursors shows homogenous phase. However, as the ladder-like silicone network is being constructed by sol-gel reaction, molecular micro-phase separation between the polymer chains and the assembling network occurs, so that thermodynamic molecular entanglement is spontaneously induced (Supplementary Fig. 1). As a result, 3-dimensional molecular entanglement between the silicone network and the polymer chains is achieved, indicating that the polymer chains are molecularly aggregated. The induced chain aggregation facilitates charge transporting capability, so that the electrical characteristics of the polymer semiconductor are not deteriorated even though insulating materials are introduced (Adv. Mater. 2019, 31, 1901400;ACS Appl. Mater. Inter. 2020, 12, 55107-55115). Recently, the molecular aggregation mechanism has been exploited in the field of polymer LEDs (Nature 2022, 603, 624-630;Adv. Mater. 2022, 34, 2201844;Sci. Adv. 2021, 7, eabd9715). These works proved the enhanced luminance efficiency when insulating materials were embedded into the light-emitting semiconductors, and elucidated that the performance improvement was attributed to enhanced charge transporting characteristics of the emission layers by the chain aggregation. This is analogous to our results shown in Supplementary Fig. 15, exhibiting higher current density and luminance in the SI-OLEDs than pristine OLEDs. Consequently, enhanced charge transport of the SI-OLES induced by molecular aggregation is responsible for the improvement EL performance, indicating the novelty of our material design in that 7 micro-patternability and light-emitting performance can be obtained simultaneously.
To clarify the origin of the improved luminance efficiency of the SI-OLEDs, we added the following sentence and references on the page 9 and References, respectively, in the revised manuscript.
"This preserved or improved light-emitting performance of the SI-OLEDs is attributed to the molecular entanglement in the SI-OLES caused by the silicone network, leading to enhanced charge transport within the emission layers 26,27,[33][34][35] ." Response: We thank the comment from the reviewer. As the reviewer mentioned, although the micropatterns of the blue (B)-OLES may appear to be plausibly well-defined, when we took a look deeply, the pattern fidelity of the B-OLES (variation of pattern width and pattern edge roughness) was significantly different from that of the SI-OLES. As shown in Figure R4, the B-OLES micropatterns (2 μm × 6 μm) exhibited much higher values of the width variation and edge roughness than the R-and G-SI-OLES. This indicates that a side-etching behavior was dominantly generated in the B-OLES, so that much smaller pattern than expected and severe side abrasion of the micropatterns were obtained. These results could be serious challenges in consecutive patterning processes and bring about low pixel uniformity. As the case of the R-and G-SI-OLES, the physico-chemical robustness of the B-OLES could be achieved by incorporating silicone network (Figure R7), but at the same time, incorporating silicone network accompanied considerable degradation in the luminescence characteristics for B-OLES (which even in its pristine form already exhibit inferior luminescence efficiency compared to pristine R-and G-OLES). Therefore, B-OLES but not the B-SI-OLES was used in this manuscript. We anticipate that development of high efficiency B-OLES would provide effective solution to realize the B-SI-OLES, thereby achieving highly reliable fine micropatterns of the B-SI-OLES.
To alleviate the reviewer's concern, we added this figure as Supplementary Figure 20 in the revised manuscript.

Reply to Reviewer #3
The manuscript entitled "Silicone engineered anisotropic lithography for ultrahigh-density OLEDs" submitted by D. H. Kim and coworkers presented intriguing etching characteristics of siliconeincorporated organic light-emitting semiconductors (SI-OLES), enabling to be delicately pixelated by conventional photolithography and reactive ion etching. The approach that unique etching behavior of silicon is implanted on the OLED materials is particularly noteworthy, and this technique seems to significantly contribute improvement of pattern resolution in the field of the OLEDs. Also, the authors systematically described the non-volatile blocking layer that is induced during etching of the SI-OLES, as well as the developed high density RGB OLED pattern arrays and the light-emitting performance were quite impressive. In addition, the manuscript is well-written, and the figures are well-organized. I recommend this high quality manuscript for publication in Nature Communications after minor revision, and several points should be addressed before publication.

Response:
We appreciate the positive comments from the reviewer. We revised the manuscript according to the reviewer's comments. network precursors shows homogenous phase; however, as the ladder-like silicone network is constructing by sol-gel reaction, molecular micro-phase separation between the polymer chains and the assembling network occurs, so that thermodynamic molecular entanglement is spontaneously induced (Supplementary Fig. 1). As a result, 3-dimensional molecular entanglement between the silicone network and the polymer chains is achieved, indicating that the polymer chains are molecularly aggregated. The induced chain aggregation facilitates charge transporting capability, so that the electrical characteristics of the polymer semiconductor are not deteriorated even though insulating materials are introduced (Adv. Mater. 2019, 31, 1901400;ACS Appl. Mater. Inter. 2020, 12, 55107-55115). Recently, the molecular aggregation mechanism has been exploited in the field of polymer LEDs (Nature 2022, 603, 624-630;Adv. Mater. 2022, 34, 2201844;Sci. Adv. 2021, 7, eabd9715). These works presented the enhanced luminance efficiency when insulating materials were embedded into the light-emitting semiconductors, and elucidated that the performance improvement was attributed to enhanced charge transporting characteristics of the emission layers by the chain aggregation. This is analogous to our results shown in Supplementary Fig. 15, exhibiting higher current density and luminance in SI-OLEDs than pristine OLEDs. Consequently, enhanced charge transporting ability of SI-OLES induced by molecular aggregation is responsible for the improvement EL performance, indicating the novelty of our material design in that micro-patternability and light-emitting performance can be obtained simultaneously.
To clarify the origin of the improved luminance efficiency of the SI-OLEDs, we added the following sentence and references on the page 9 and References, respectively, in the revised manuscript.
"This preserved or improved light-emitting performance of the SI-OLEDs is attributed to the molecular entanglement in the SI-OLES caused by the silicone network, leading to enhanced charge transport within the emission layers 26,27,[33][34][35] ."

Q4) The author mentioned that the used Blue-OLES in this work was not incorporated with silicone network because the luminescence performance of the SI-B-OLES was too low, and this problem can be solved if high efficient Blue-OLES is exploited. However, even if high efficient blue materials are developed, it could be meaningless if the chemical robustness of bluish semiconducting materials is not
demonstrated when silicone network is adapted. At least, the authors should provide additional data that even blue materials can possess high tolerance when silicone network is integrated.

Response:
We appreciate the comment from the reviewer. As the reviewer mentioned, we carried out further experiments to investigate whether the adapted B-OLES in this work can obtain physico-chemical tolerance when the silicone network is embedded. As shown in Figure R7, the B-SI-OLES film showed almost 100 % film retention after solvent rinsing process. In conjunction with the chemical robustness, the etch rate of the B-SI-OLES was diminished by 21.7 %, compared to that of the B-OLES. These results indicate that the proposed SI-OLES material design can be also effective methodology to impart chemical and physical robustness into blue light-emitting polymer series. Consequently, we believe that our material design possesses a great potential to realize ultrahigh-density blue OLEDs with high light-emitting performance when high efficiency blue materials are developed.
To address the reviewer's concern, we added this figure as Supplementary Figure 19 and the following sentence on the page 11 in the revised manuscript.
"Note that unlike the R-and G-SI-OLES, the B-OLES was not integrated with the silicone network in this study, even though the B-OLES was able to achieve high physico-chemical tolerance when the silicone network was embedded (Supplementary Fig. 19). This is because the B-OLES is based on a ternary 12 blending system (host-dopant-hole transporting materials) to enhance its luminescence efficiency; however, the incorporated silicone network seems to interfere energy transfer process within the B-OLES layer, so that the light-emitting performance significantly deteriorated."

Response:
We appreciate the reviewer for pointing out typing errors. We corrected the typos and reflected them in the revised manuscript.